Simulation of inter-connected power systems can be achieved with distributed processing， which coordinates computing and data resources dispersed in different dispatching centers. Nevertheless， computing process may be interfered by various uncertain disturbances， because the process must be conducted via wide area communication network. Therefore， it is necessary to study the convergence and correctness of distributed simulations subjected to disturbances，namely， robustness of distributed processing for power system simulation. In this paper， the definition for robustness of distributed processing is proposed first. A method is presented to study the robustness of distributed processing which subjected to communication error. Furthermore， a distributed power flow calculating process is used to illustrate the method and the result demonstrates its universality.